Planning the IP-Based Infrastructure

To create or expand an enterprise network, you can choose from many design models, including a network infrastructure model based on the three-tier design model. This model, a hierarchical network de 323p159d sign model described by Cisco Systems, Inc. and other networking vendors, is widely used as a reference in the design of enterprise networks.

Figure 1.2 shows the tasks involved in creating a three-tier TCP/IP infrastructure.

Figure 1.    Planning the IP-Based Infrastructure

The modular nature of a hierarchical model such as the three-tier model can simplify deployment, capacity planning, and troubleshooting in a large internetwork. In this design model, the tiers represent the logical layers of functionality within the network. In some cases, network devices serve only one function; in other cases, the same device may function within two or more tiers.

The three tiers of this hierarchical model are referred to as the core, distribution, and access tiers. Figure 1.3 illustrates the relationship between network devices operating within each tier.

Figure 1.    Three-Tier Network Design Model

Designing the Access Tier

The access tier is the layer in which users connect to the rest of the network, including individual workstations and workgroup servers. The access tier usually includes a relatively large number of low- to medium-speed access ports, whereas the distribution and core tiers usually contain fewer, but higher-speed network ports. Design the access tier with efficiency and economy in mind, and balance the number and types of access ports to keep the volume of access requests within the capacity of the higher layers.

Designing the Distribution Tier

The distribution tier distributes network traffic between related access layers, and separates the locally destined traffic from the network traffic destined for other tiers through the core.

Network security and access control policies are often implemented within this tier. Network devices in this layer can incorporate technologies such as firewalls and address translators.

The distribution tier is often the layer in which you define subnets; through the definition of subnets, distribution devices often function as routers. Decisions about routing methods and routing protocols affect the scalability and performance of the network in this tier.

A server network in the distribution layer might house critical network services and centralized application servers. Computers running Windows Server 2003 can be used there to run the Active Directory directory service, DNS, DHCP, and other core infrastructure services.

Designing the Core Tier

The core tier facilitates the efficient transfer of data between interconnected distribution tiers. The core tier typically functions as the high-speed backbone of the enterprise network. This tier can include one or more building-wide or campus-wide backbone local area networks (LANs), metropolitan area network (MAN) backbones, and high-speed regional wide area network (WAN) backbones.

The primary design goal for the core is reliable, high-speed network performance. As a general rule, locate any feature that might affect the reliability or performance of this tier in an access or distribution tier instead.

Select highly reliable network equipment for the core tier, and design a fault-tolerant core system whenever possible. Many products meet these criteria, and most major network vendors offer complete solutions to meet the requirements of the core tier.

For more information about designing a three-tier network model, see "Additional Resources" later in this chapter.